Project Summary: Metastatic breast cancer (MBC) is responsible for 90% of patient breast cancer deaths with no curative therapies currently available. Our discovery, as shown in our preliminary data, of an abscopal effect following nanosecond electric pulse (nsEP) tumor treatment reveals a promising novel immunotherapeutic approach to control or treat MBC in conjunction with the local nsEP tumor ablation. Currently nsEPs, an electrical engineering technology, have been developed and utilized to treat cancer in animal models. Preclinical studies on nsEP applications have focused mainly on local tumor ablation, however, a concurrent vaccine-like protective effect has been reported in the literature in an orthotopic hepatocellular carcinoma model following nsEP treatment. Recently, we've discovered an abscopal effect resulting from local tumor ablation by nsEPs in an orthotopic murine breast cancer model. Our preliminary data showed nsEP tumor ablation not only ablated the local tumor but also prevented spontaneous organ metastases and rejected existing distant tumors. Furthermore, we demonstrated remarkable changes of immune profiles of tumor animals treated with nsEPs where extremely suppressive local and systemic immune environments in tumor-bearing animals were diminished after successful nsEP treatment. Meanwhile, central and effector memory T cells were established, exhibiting cytotoxic functions. Our data also suggest the efficacy of nsEPs for tumor regression could be further improved in combination with other immunomodulators. Based on the above preliminary findings, we hypothesize that the abscopal effect following nsEP tumor ablation is due to antitumor immunity induced by nsEP treatment with the potential to be enhanced with other immunomodulators. Two specific aims will be addressed to test our hypothesis. In Aim 1, we will evaluate: (a) the influence of nsEP parameters on the abscopal effect, (b) the strength of the abscopal effect, and (c) the enhancement of the abscopal effect with immunomodulators targeting major immune defects of the tumor. In Aim 2, we will explore the immune mechanisms of the abscopal effect resulting from nsEP tumor ablation. We will characterize the change of tumor microenvironment, local and systemic immune responses following nsEP tumor ablation, and determine the role of innate or adaptive immunity for the abscopal effect. The nsEP technology is a drug-free, ionizing radiation-free, and non-thermal therapy that can treat cancer in a fast and minimally invasive manner. Successful results from this study would translate to a breakthrough tumor regression or potentially curative therapy for metastatic breast cancer.